Process for reducing sheeting during polymerization of alpha-olefins

ABSTRACT

A process for reducing sheeting during production of polyolefins by polymerization of alpha-olefins utilizing titanium based polymerization catalysts wherein the static electric charges in the reactor at the side of possible sheet formation are maintained below static voltage levels which would otherwise cause sheet formation.

This application is a divisional of application Ser. No. 730,958 filedMay 6, 1985, now U.S. Pat. No. 4,792,592, and which is a continuation ofapplication Ser. No. 650,571 filed Sept. 14, 1984, now U.S. Pat. No.4,532,311, and which is a continuation of Ser. No. 247,990 filed Mar.26, 1981 which is now abandoned.

Conventional low density polyethylene has been historically polymerizedin heavy walled autoclaves or tubular reactors at pressures as high as50,000 psi and temperatures up to 300° C. or higher. The molecularstructure of high pressure, low density polyethylene (HP-LDPE) is highlycomplex. The permutations in the arrangement of their simple buildingblocks are essentially infinite. HP-LDPE's are characterized by anintricate long chain branched molecular architecture. These long chainbranches have a dramatic effect on the melt rheology of these resins.HP-LDPE's also possess a spectrum of short chain branches, generally 1to 6 carbon atoms in length. These short chain branches disrupt crystalformation and depress resin density.

More recently, technology has been provided whereby low densitypolyethylene can be produced by fluidized bed techniques at lowpressures and temperatures by copolymerizing ethylene with various alphaolefins. These low pressure LDPE (LP-LDPE) resins generally possesslittle, if any, long chain branching and are sometimes referred to aslinear LDPE resins. They are short chain branched with branch length andfrequency controlled by the type and amount of comonomer used duringpolymerization.

As is well known to those skilled in the art, low pressure, high or lowdensity polyethylenes can now be conventionally provided by a fluidizedbed process utilizing several families of catalysts to produce a fullrange of low density and high density products. The appropriateselection of catalysts to be utilized depends in part upon the type ofend product desired, i.e., high density, low density, extrusion grade,film grade resins and other criteria.

The various types of catalysts which may be used to producepolyethylenes in fluid bed reactors can generally be typed as follows:

Type I. The silyl chromate catalysts disclosed in U.S. Pat. No.3,324,101 to Baker and Carrick and U.S. Pat. No. 3,324,095 to Carrick,Karapinka and Turbet. The silyl chromate catalysts are characterized bythe presence therein of a group of the formula: ##STR1## wherein R is ahydrocarbyl group having from 1 to 14 carbon atoms. The preferred silylchromate catalysts are the bis(triarylsilyl) chromates and morepreferably bis(triphenysilyl) chromate.

This catalyst is used on a support such as silica, alumina, thoria,zirconia and the like, other supports such as carbon black,micro-crystalline cellulose, the non-sulfonated ion exchange resins andthe like may be used.

Type II. The bis(cyclopentadienyl) chromium (II) compounds disclosed inU.S. Pat. No. 3,879,368. These bis(cyclopentadienyl) chromium (II)compounds have the following formula: ##STR2## wherein R' and R" may bethe same or different C₁ to C₂₀, inclusive, hydrocarbon radicals, and n'and n" may be the same or different integers of 0 to 5, inclusive. TheR' and R" hydrocarbon radicals may be saturated or unsaturated, and caninclude aliphatic, alicyclic and aromatic radicals such as methyl,ethyl, propyl, butyl, pentyl, cyclopentyl, cyclohexyl, allyl, phenyl andnaphthyl radicals.

These catalysts are used on a support as heretofore described.

Type III. The catalysts as described in U.S. Pat. No. 4,011,382. Thesecatalysts contain chromium and titanium in the form of oxides and,optionally, fluorine and a support. The catalysts contain, based on thecombined weight of the support and the chromium, titanium and fluorine,about 0.05 to 3.0, and preferably about 0.2 to 1.0, weight percent ofchromium (calculated as Cr), about 1.5 to 9.0, and preferably about 4.0to 7.0, weight percent of titanium (calculated as Ti), and 0.0 to about2.5, and preferably about 0.1 to 1.0 weight percent of fluorine(calculated as F).

The chromium compounds which may be used for the Type III catalystsinclude CrO₃, or any compound of chromium which is oxidizable to CrO₃under the activation conditions employed. At least a portion of thechromium in the supported, activated catalyst must be in the hexavalentstate. Chromium compounds other than CrO₃ which may be used aredisclosed in U.S. Pat. No. 2,825,721 and U.S. Pat. No. 3,622,521 andinclude chromic acetyl acetonate, chromic nitrate, chromic acetate,chromic chloride, chromic sulfate, and ammonium chromate.

The titanium compounds which may be used include all those which areoxidizable to TiO₂ under the activation conditions employed, and includethose disclosed in U.S. Pat. No. 3,622,521 and Netherlands PatentApplication 72-10881.

The fluorine compounds which may be used include HF, or any compound offluorine which will yield HF under the activation conditions employed.Fluorine compounds other than HF which may be used are disclosed inNetherlands Patent Application 72-10881.

The inorganic oxide materials which may be used as a support in thecatalyst compositions are porous materials having a high surface area,that is, a surface area in the range of about 50 to about 1000 squaremeters per gram, and an average particle size of about 20 to 200microns. The inorganic oxides which may be used include silica, alumina,thoria, zirconia and other comparable inorganic oxides, as well asmixtures of such oxides.

Type IV. The catalysts as described in U.S. patent application, Ser. No.892,325, filed on Mar. 31, 1978, in the names of F. J. Karol et al, andentitled, "Preparation of Ethylene Copolymers in Fluid Bed Reactor" andassigned to the same assignee as the present application. Thesecatalysts comprise at least one titanium compound, at least onemagnesium compound, at least one electron donor compound, at least oneactivator compound and at least one inert carrier material.

The titanium compound has the structure

    Ti(OR).sub.a X.sub.b

wherein R is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical, orCOR' where R' is a C₁ to C₁₄ aliphatic or aromatic hydrocarbon radical;X is Cl, Br, or I; a is 0 or 1; b is 2 to 4 inclusive; and a+b=3 or 4.

The titanium compounds can be used individually or in combinationthereof, and would include TiCl₃, TiCl₄, Ti(OCH₃)Cl₃, Ti(OC₆ H₅)Cl₃,Ti(OCOCH₃)Cl₃ and Ti(OCOC₆ H₅)Cl₃.

The magnesium compound has the structure

    MgX.sub.2

wherein X is Cl, Br, or I. Such magnesium compounds can be usedindividually or in combination thereof and would include MgCl₂, MgBr₂and MgI₂. Anhydrous MgCl₂ is the preferred magnesium compound.

The titanium compound and the magnesium compound are generally used in aform which will facilitate their dissolution in the electron donorcompound.

The electron donor compound is an organic compound which is liquid at25° C. and in which the titanium compound and the magnesium compound arepartially or completely soluble. The electron donor compounds are knownas such or as Lewis bases.

The electron donor compounds would include such compounds as alkylesters of aliphatic and aromatic carboxylic acids, aliphatic ethers,cyclic ethers and aliphatic ketones.

The catalyst may be modified with a boron halide compound having thestructure

    BR.sub.c X'.sub.3-c

wherein

R is an aliphatic or aromatic hydrocarbon radical containing from 1 to14 carbon atoms or OR', wherein R' is also an aliphatic or aromatichydrocarbon radical containing from 1 to 14 carbon atoms,

X' is selected from the group consisting of Cl and Br, or mixturesthereof, and

c is 0 or 1 when R is an aliphatic or aromatic hydrocarbon and 0, 1 or 2when R is OR'.

The boron halide compounds can be used individually or in combinationthereof, and would include BCl₃, BBr₃, B(C₂ H₅)Cl₂, B(OC₂ H₅)Cl₂, B(OC₂H₅)₂ Cl, B(C₆ H₅)Cl₂, B(OC₆ H₅)Cl₂, B(C₆ H₁₃)Cl₂, B(OC₆ H₁₃)Cl₂, andB(OC₆ H₅)₂ Cl. Boron trichloride is the particularly preferred boroncompound.

The activator compound has the structure

    Al(R").sub.c X'.sub.d H.sub.e

wherein X' is Cl or OR₁ ; R₁ and R" are the same or different and are C₁to C₁₄ saturated hydrocarbon radicals, d is 0 to 1.5, e is 1 or 0, andc+d+e=3.

Such activator compounds can be used individually or in combinationsthereof.

The carrier materials are solid, particulate materials and would includeinorganic materials such as oxides of silicon and aluminum and molecularsieves, and organic materials such as olefin polymers, e.g.,polyethylene.

In general, the above catalysts are introduced together with thepolymerizable materials, into a reactor having an expanded section abovea straight-sided section. Cycle gas enters the bottom of the reactor andpasses upward through a gas distributor plate into a fluidized bedlocated in the straight-sided section of the vessel. The gas distributorplate serves to ensure proper gas distribution and to support the resinbed when gas flow is stopped.

Gas leaving the fluidized bed entrains resin particles. Most of theseparticles are disengaged as the gas passes through the expanded sectionwhere its velocity is reduced.

The operating difficulties associated with the utilization of catalysttypes I through III in the above described reactors have beensubstantially eliminated, resulting in the economic and efficientproduction of low pressure, low or high density polyethylene resinswhich have a wide variety of uses.

In order to satisfy certain end use applications or ethylene resins,such as for film, injection molding and roto molding applications,catalyst type IV has been used. However, attempts to produce certainethylene resins utilizing the type IV catalysts supported on a poroussilica substrate in certain fluid bed reactors, have not been entirelysatisfactory from a practical commercial standpoint. This is primarilydue to the formation of "sheets" in the reactor after a brief period ofoperation. The "sheets" can be characterized as constituting a fusedpolymeric material.

The sheets vary widely in size, but are similar in most respects. Theyare usually about 1/4 to 1/2 inch thick and are from about one to fivefeet long, with a few specimens even longer. They have a width of about3 inches to more than 18 inches. The sheets have a core composed offused polymer which is oriented in the long direction of the sheets andtheir surfaces are covered with granular resin which has fused to thecore. The edges of the sheets have a hairy appearance from strands offused polymer.

After a relatively short period of time during polymerization, sheetsbegin to appear in the reactor, and these sheets plug product dischargesystems forcing shutdown of the reactor.

Accordingly, it will be seen that there presently exists a need toimprove the polymerization techniques necessary for the production ofpolyolefin products utilizing titanium based catalysts in fluidized bedreactors.

It is therefore an object of the present invention to provide a processto substantially reduce or eliminate the amount of sheeting which occursduring the low pressure fluidized bed polymerization of alpha olefinsutilizing titanium based compounds as catalyst.

Another object is to provide a process for treating fluidized bedreactors utilized for the production of polyolefin resins utilizingtitanium based catalyst or other catalysts which result in similarsheeting phenomena.

These and other objects will become readily apparent from the followingdescription taken in conjunction with the accompanying drawing whichgenerally indicates a typical gas phase fluidized bed polymerizationprocess for producing high density and low density polyolefins.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE of the drawing illustrates a conventional fluidized bedreaction system for polymerizing alpha-olefins.

Broadly contemplated, the present invention provides an improvement inthe method for polymerization of alpha olefins in a fluid bed reactorutilizing titanium based catalysts or other catalysts prone to causesheeting during said polymerization, the improvement comprisingmaintaining the static electric charge in said reactor at the site ofpossible sheet formation below static voltage levels which wouldotherwise cause sheet formation.

The critical static voltage level for sheet formation is a complexfunction of resin sintering temperature, operating temperature, dragforces in the fluid bed, resin particle size distribution and recyclegas composition. The static voltage can be reduced by a variety oftechniques such as by treating the reactor surface to reduce staticelectric generation, by injection of an antistatic agent to increaseparticle surface electrical conductivity thus promoting particledischarging; by installation of appropriate devices connected to thereactor walls which are designed to promote electrical discharging bycreating areas of high localized field strength, and by neutralizationof charges by the injection or creation of ion pairs, ions or chargedparticles of the opposite polarity from the resin bed.

A particularly preferred technique generally involves treating thereactor vessel prior to polymerization by introducing a chromiumcontaining compound into the reaction vessel in a non-reactingatmosphere.

Referring particularly to the sole FIGURE of the drawing, a conventionalfluidized bed reaction system for polymerizing alpha-olefins includes areactor 10 which consists of a reaction zone 12 and a velocity reductionzone 14.

The reaction zone 12 includes a bed of growing polymer particles, formedpolymer particles and a minor amount of catalyst particles fluidized bythe continuous flow of polymerizable and modifying gaseous components inthe form of make-up feed and recycle gas through the reaction zone. Tomaintain a viable fluidized bed, the mass gas flow rate through the bedis normally maintained above the minimum flow required for fluidization,and preferably from about 1.5 to about 10 times G_(mf) and morepreferably from about 3 to about 6 times G_(mf). G_(mf) is used in theaccepted form as the abbreviation for the minimum gas flow required toachieve fluidization, C. Y. Wen and Y. H. Yu, "Mechanics ofFluidization", Chemical Engineering Progress Symposium Series, Vol. 62,p. 100-111 (1966).

It is highly desirable that the bed always contains particles to preventthe formation of localized "hot spots" and to entrap and distribute theparticulate catalyst throughout the reaction zone. On start up, thereactor is usually charged with a base of particulate polymer particlesbefore gas flow is initiated. Such particles may be identical in natureto the polymer to be formed or different therefrom. When different, theyare withdrawn with the desired formed polymer particles as the firstproduct. Eventually, a fluidized bed of the desired polymer particlessupplants the start-up bed.

The appropriate catalyst used in the fluidized bed is preferably storedfor service in a reservoir 16 under a blanket of a gas which is inert tothe stored material, such as nitrogen or argon.

Fluidization is achieved by a high rate of gas recycle to and throughthe bed, typically in the order of about 50 times the rate of feed ofmake-up gas. The fluidized bed has the general appearance of a densemass of viable particles in possible free-vortex flow as created by thepercolation of gas through the bed. The pressure drop through the bed isequal to or slightly greater than the mass of the bed divided by thecross-sectional area. It is thus dependent on the geometry of thereactor.

Make-up gas is fed to the bed at a rate equal to the rate at whichparticulate polymer product is withdrawn. The composition of the make-upgas is determined by a gas analyzer 18 positioned above the bed. The gasanalyzer determines the composition of the gas being recycled and thecomposition of the make-up gas is adjusted accordingly to maintain anessentially steady state gaseous composition within the reaction zone.

To insure complete fluidization, the recycle gas and, where desired,part or all of the make-up gas are returned to the reactor at base 20below the bed. Gas distribution plate 22 positioned above the point ofreturn ensures proper gas distribution and also supports the resin bedwhen gas flow is stopped.

The portion of the gas stream which does not react in the bedconstitutes the recycle gas which is removed from the polymerizationzone, preferably by passing it into velocity reduction zone 14 above thebed where entrained particles are given an opportunity to drop back intothe bed.

The recycle gas is then compressed in a compressor 24 and thereafterpassed through a heat exchanger 26 wherein it is stripped of heat ofreaction before it is returned to the bed. By constantly removing heatof reaction, no noticeable temperature gradient appears to exist withinthe upper portion of the bed. A temperature gradient will exist in thebottom of the bed in a layer of about 6 to 12 inches, between thetemperature of the inlet gas and the temperature of the remainder of thebed. Thus, it has been observed that the bed acts to almost immediatelyadjust the temperature of the recycle gas above this bottom layer of thebed zone to make it conform to the temperature of the remainder of thebed thereby maintaining itself at an essentially constant temperatureunder steady conditions. The recycle is then returned to the reactor atits base 20 and to the fluidized bed through distribution plate 22. Thecompressor 24 can also be placed downstream of heat exchanger 26.

Hydrogen may be used as a chain transfer agent for conventionalpolymerization reactions of the types contemplated herein. In the casewhere ethylene is used as a monomer the ratio of hydrogen/ethyleneemployed will vary between about 0 to about 2.0 moles of hydrogen permole of the monomer in the gas stream.

Any gas inert to the catalyst and reactants can also be present in thegas stream. The cocatalyst is added to the gas recycle stream upstreamof its connection with the reactor as from dispenser 28 through line 30.

As is well known, it is essential to operate the fluid bed reactor at atemperature below the sintering temperature of the polymer particles.Thus to insure that sintering will not occur, operating temperaturesbelow sintering temperature are desired. For the production of ethylenepolymers an operating temperature of from about 90° to 100° C. ispreferably used to prepare products having a density of about 0.94 to0.97 while a temperature of about 75° to 95° C. is preferred forproducts having a density of about 0.91 to 0.94.

Normally the fluid bed reactor is operated at pressures of up to about1000 psi, and is preferably operated at a pressure of from about 150 to350 psi, with operation at the higher pressures in such ranges favoringheat transfer since an increase in pressure increases the unit volumeheat capacity of the gas.

The catalyst is injected into the bed at a rate equal to its consumptionat a point 32 which is above the distribution plate 22. A gas which isinert to the catalyst such as nitrogen or argon is used to carry thecatalyst into the bed. Injecting the catalyst at a point abovedistribution plate 22 is an important feature. Since the catalystsnormally used are highly active, injection into the area below thedistribution plate may cause polymerization to begin there andeventually cause plugging of the distribution plate. Injection into theviable bed, instead, aids in distributing the catalyst throughout thebed and tends to preclude the formation of localized spots of highcatalyst concentration which may result in the formation of "hot spots".

Under a given set of operating conditions, the fluidized bed ismaintained at essentially a constant height by withdrawing a portion ofthe bed as product at a rate equal to the rate of formation of theparticulate polymer product. Since the rate of heat generation isdirectly related to product formation, a measurement of the temperaturerise of the gas across the reactor (the difference between inlet gastemperature and exit gas temperature) is determinative of the rate ofparticulate polymer formation at a constant gas velocity.

The particulate polymer product is preferably withdrawn at a point 34 ator close to distribution plate 22. The particulate polymer product isconveniently and preferably withdrawn through the sequential operationof a pair of timed valves 36 and 38 defining a segregation zone 40.While valve 38 is closed, valve 36 is opened to emit a plug of gas andproduct to the zone 40 between it and valve 36 which is then closed.Valve 38 is then opened to deliver the product to an external recoveryzone and after delivery, valve 38 is then closed to await the nextproduct recovery operation.

Finally, the fluidized bed reactor is equipped with an adequate ventingsystem to allow venting the bed during the start up and shut down. Thereactor does not require the use of stirring means and/or wall scrapingmeans.

The reactor vessel is normally constructed of carbon steel and isdesigned for the operating conditions stated above.

In order to better illustrate the problems incident to the utilizationof the type IV catalysts, reference is again made to the drawing. Thetitanium based catalyst (type IV) is introduced into the reactor 10 atpoint 32. Under conventional operations on certain resins, after a briefperiod of time, i.e. in the order of about 36 to 72 hours, sheets beingto form in reactor 10, at a site in the reactor proximate the wall ofthe reactor and located about a distance of one-half the reactordiameter up from the base of the fluid bed. The sheets of fused resinbegin to appear in segregation zone 40, rapidly plugging the system,causing the reactor to be shut down. More characteristically thesheeting begins after production equivalent to 6 to 10 times the weightof the bed of resin in reactor 10.

Many possible causes were investigated in attempting to discover andeliminate the sheeting. In the course of the investigation,thermocouples were installed just inside the reactor walls at elevationsof 1/4 and 1/2 reactor diameter above the gas distributor plate. Underconventional operations, "skin" thermocouples indicate temperaturesequal to the temperature of the fluidized bed. When sheeting occurs,these thermocouples indicate temperature excursions of up to 20° C.above the temperature of the fluidized bed thus providing reliableindication of the occurrence of sheeting. In addition, an electrostaticvoltmeter was used to measure voltage on a 1/2 inch spherical electrodelocated in the fluid bed 1 inch radially from the reactor wall and 1/2reactor diameter above the gas distributor plate. The location wasselected because sheet formation was observed to initiate in a bandranging from 1/4 to 3/4 reactor diameter in elevation above the base ofthe fluid bed. As is well known for deep fluidized beds, thiscorresponds to the region of least mixing intensity near the wall, i.e.a null zone where particle motion near the wall changes from generallyupward to generally downward. The possible causes investigated includedfactors affecting mixing in the fluidized bed, reactor operatingconditions, catalyst and resin particle size, particle sizedistribution, and others. A correlation was found between sheeting andbuildup of static electric charge on the resin particles proximate thereactor walls. When the static voltage level of resin particles atparticular sites proximate the reactor wall in a fluidized bed reactoris low, the reactor runs normally and no sheets are formed. When thestatic voltage level exceeds a critical level at those sites,uncontrolled sheeting occurs and the reactor must be shut down.

Surprisingly sheeting had not occurred to any significant degree on anyresin utilizing the type IV catalysts in reactors which had previouslyutilized type II catalysts or in reactors that had utilized type Ithrough III catalysts.

It was further discovered that sheeting could be substantially reducedand in some cases entirely eliminated by controlling static voltage inthe fluidized bed at a site proximate the reactor walls below thecritical level for sheet formation. This critical level for sheetformation is not a fixed value, but is a complex function dependent onvariables including resin sintering temperature, operating temperature,drag forces in the fluid bed, resin particle size distribution andrecycle gas composition.

The critical voltage level Vc for sheeting of ethylene homopolymers andethylene-butene copolymers is primarily a function of the resinsintering temperature, the reactor bed temperature and the concentrationof hydrogen in the recycle gas. The relationship can be expressed as:

    Vc=-8000-50 Ts+90[H.sub.2 ]+150To

where

Vc=voltage below which sheeting will not occur, volts;

Ts=sintering temperature of resin under reactor operating conditions, in°C.;

To=temperature of reactor, in °C. and

[H₂ ]=mole percent hydrogen in recycle gas

The sintering temperature of the resin under reactor operatingconditions is the temperature at which a settled bed of resin in contactwith a gas having the same composition as the reactor recycle gas usedin producing the resin will sinter and form aggolomerates whenrefluidization is attempted after allowing the bed to remain settled forfifteen minutes. The sintering temperature is decreased by decreasingthe resin density, by increasing the melt index and by increasing theamount of dissolved monomers.

The constants in the equation were determined from data collected duringreactor operation when the reactor just began to exhibit sheetingsymptoms through skin thermocouple temperature excursions above the bedtemperature. The voltage indicated on the voltage probe describedearlier varies with time due to the random nature of a fluidized bed.Thus the critical voltage, Vc, is expressed as a time averaged voltage.Voltage measurements are difficult to interpret because additionalstatic electric charge is generated when a sheet, formed because of astatic charge, separates from the reactor wall. In addition, thesheeting phenomena can start as a very local phenomenon and spreadfurther clouding interpretation of voltage readings.

Although the sheeting mechanism is not fully understood, it is believedthat static electricity generated in the fluid bed charges resinparticles. When the charge on the particles reaches the level where theelectrostatic forces trying to hold the charged particle near thereactor wall exceed the drag forces in the bed trying to move theparticle away from the wall, a layer of catalyst containing,polymerizing resin particles forms a non-fluidized layer near thereactor wall. Heat removal from this layer is not sufficient to removethe heat of polymerization because the non-fluidized layer near the wallhas less contact with the fluidizing gas than do particles in thefluidized portion of the bed. The heat of polymerization increases thetemperature of the nonfluidized layer near the reactor wall until theparticles melt and fuse. At this point other particles from thefluidized bed will stick to the fused layer and it will grow in sizeuntil it comes loose from the reactor wall. The separation of adielectric from a conductor (the sheet from the reactor wall) is knownto generate additional static electricity thus accelerating subsequentsheet formation.

The art teaches various processes whereby static voltage can be reducedor eliminated. These comprise (1) reducing the rate of chargegeneration, (2) increasing the rate of discharge of electrical charge,and (3) neutralization of electrical charge. Some processes suited foruse in a fluidized bed comprise (1) use of an additive to increase theconductivity of the particles thus providing a path for discharging, (2)installation of grounding devices in a fluidized bed to provideadditional area for discharging electrostatic charges to ground, (3)ionization of gas or particles by electrical discharge to generate ionsto neutralize electrostatic charges on the particles, and (4) the use ofradioactive sources to produce radiation that will create ions toneutralize electrostatic charges on the particles. The application ofthese techniques to a commercial scale, fluidized bed, polymerizationreactor may not be feasible or practical. Any additive used must not actas a poison to the polymerization catalyst and must not adversely affectthe quality of the product. Thus water, the most widely used additive toreduce static on particles, cannot be used since it is a severe catalystpoison. The installation of grounding devices may actually generateadditional electrostatic charge since the friction of resin particles onmetal surfaces creates electrostatic charges on the resin particles. Theuse of ion generators and radiation sources pose severe problems ofscale. The ions generated by electric discharge or radiation will beattracted to the reactor walls and other grounded objects and willtravel only a limited distance before contacting a grounded object. Thusthe ions may not travel far enough from the site of ion generation todischarge the region of the bed where sheeting occurs. Generation ofions within the fluid bed is severely limited by the quenching effect ofthe cloud of charged particles which form around an ion generator. Thusthe number of ion generation sources required may be high adding to thecomplexity and danger of radiation sources or electrical dischargegenerators in or near a pressurized, hydrocarbon containing reactor. Inthe course of the investigation, it was discovered that an effectiveprocess for treating the walls of the reactor vessel to reduce staticcharge generation comprises operation of the reactor for a short, i.e.two week, period utilizing a chromium containing catalyst (type Ithrough III) where the chromium is in the 2 or 3 valence state during atleast part of its residence time in the reactor.

Surprisingly, however, it was also discovered that if the walls of thereactor vessel are treated prior to the commencement of polymerizationwith a chromium containing compound wherein the chromium is present inthe reactor at a valence of 2 or 3, then the formation of sheetingduring polymerization is substantially reduced and in some casesentirely eliminated.

The chromium containing compounds contemplated for use in the presentinvention are as explained previously those in which the chromium ispresent in the reactor at a valence of 2 or 3. Merely as illustrativethe following compounds would be suitable for the present invention:

The bis(cyclopentadienyl) chromium (II) compounds having the followingformula: ##STR3## wherein R' and R" may be the same or different C₁ toC₂₀, inclusive, hydrocarbon radicals, and n' and n" may be the same ordifferent integers of 0 to 5, inclusive. The R' and R" hydrocarbonradicals can be saturated or unsaturated, and can include aliphatic,alicyclic and aromatic radicals such as methyl, ethyl, propyl, butyl,pentyl, cyclopentyl, cyclohexyl, allyl, phenyl and naphthyl radicals.Other specific compounds which are suitable include chromic acetylacetonate, chromic nitrate, chromous or chromic acetate, chromous orchromic chloride, chromous or chromic bromide, chromous or chromicfluoride, chromous or chromic sulfate, and polymerization catalystsproduced from chromium compounds where the chrome is in the plus 2 or 3valence state.

Bis(cyclopentadienyl) chromium(chromocene) is the preferred chromiumcontaining compound because of the excellent results achieved.

In general, the chromium containing compound is introduced into thereactor prior to polymerization and can be introduced in any manner suchthat the surface of the walls of the reactor is contacted with thechromium compound.

In a preferred technique, the chromium compound is dissolved in asuitable solvent and is introduced into the reactor in an inert ornon-reactive atmosphere. A resin bed may be employed to help dispersethe chromium compound through the reactor.

Suitable solvents for this purpose include but are not limited tobenzene, toluene, isopentane, hexane and water. The choice and use of asolvent is dependent on the form of the chrome containing compound andthe method of application selected. The function of the solvent is tocarry and aid in the dispersion of the chrome containing compound.Suitable inert or nonreactive gases include but are not limited tonitrogen, carbon dioxide, methane, ethane and air.

The amount of chromium compound utilized in the process should besufficient to effect the desired result, and the amount can be generallydetermined by one skilled in the art. In general, however, an amount ofat least 3.5×10⁻⁷ pound moles chromium per square foot of surface to betreated, preferably 1.0×10⁻⁶ to about 5×10⁻⁵ pound moles per square footof surface to be treated is preferred.

The polymers to which the present invention is primarily directed andwhich cause the sheeting problems above referred to in the presence oftitanium catalysts are linear homopolymers of ethylene or linearcopolymers of a major mol percent (≧90%) of ethylene, and a minor molpercent (≦10%) of one or more C₃ to C₈ alpha olefins. The C₃ to C₈ alphaolefins should not contain any branching on any of their carbon atomswhich is closer than the fourth carbon atom. The preferred C₃ to C₈alpha olefins are propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, heptene-1 and octene-1. This description is notintended to exclude the use of this invention with alpha olefinhomopolymer and copolymer resins in which ethylene is not a monomer.

The homopolymers and copolymers have a density ranging from about 0.97to 0.91. The density of the copolymer, at a given melt index level isprimarily regulated by the amount of the C₃ to C₈ comonomer which iscopolymerized with the ethylene. Thus, the addition of progressivelylarger amounts of the comonomers to the copolymers results in aprogressive lowering of the density of the copolymer. The amount of eachof the various C₃ to C₈ comonomers needed to achieve the same resultwill vary from monomer to monomer, under the same reaction conditions.In the absence of the comonomer, the ethylene would homopolymerize.

The melt index of a homopolymer or copolymer is a reflection of itsmolecular weight. Polymers having a relatively high molecular weight,have relatively high viscosities and low melt index.

In a typical mode of utilizing the subject invention to reduce sheeting,a reactor vessel such as shown in FIG. 1 and which is susceptible tosheeting problems by the polymerization of the above described materialsutilizing type IV catalysts is partially filled with granularpolyethylene resin which is purged with a non-reactive gas such asnitrogen and is fluidized by circulating said non-reacting gas throughthe reactor at a velocity above the minimum fluidizing velocity (Gmf) ofthe granular polyethylene and preferably at 3 to 5 Gmf. It is to beunderstood that the use of a fluidized bed of resin is a convenience insaid process and is not essential to the process. While the non-reactivegas is being circulated, a chromium-containing compound such aschromocene either neat or preferably dissolved in an inert solvent suchas toluene is introduced into the reactor. The concentration of thechromium-containing chemical in the inert solvent is not critical to theprocess but can be selected by one skilled in the art so as to assurethat the chromium-containing chemical is completely dissolved in thesolvent. For the preferred case, a solution containing 6 to 8 percent byweight of chromocene in toluene is typical. Approximately 4.0×10⁻⁵ poundmoles of the chromium-containing chemical is injected into the reactorfor every square foot of surface to be treated. The non-reacting gas iscirculated to bring the chromium-containing chemical in contact with themetal surfaces in the system. The treatment is carried out forsufficient time to achieve the desired result, typically several hoursto several days. In other modes of treatment, the chemical solutioncould be applied to the metal surfaces by painting, spraying, or otherapplication methods familiar to one skilled in the art. After treatmentthe reactor is now ready to begin polymerization in the usual manner.

Having set forth the general nature of the invention, the followingexamples illustrate some specific embodiments of the invention. It is tobe understood, however, that this invention is not limited to theexamples, since the invention may be practiced by the use of variousmodifications.

Examples 1-8 were conducted in a fluidized bed reactor as described inFIG. 1. The catalyst used was a Ziegler type, titanium based catalystsupported on porous silica produced as described earlier as type IV. Thecocatalyst used was triethyl aluminum. The products made in the exampleswere copolymers of ethylene and 1-butene. Hydrogen was used as a chaintransfer agent to control the melt index of the polymer. The reactors ofExample 1 and 2, had not been used to produce polyethylene with anycatalyst except those of the type described earlier as type IV.

EXAMPLE 1

A fluidized bed reactor was started up at operating conditions designedto produce a film grade low density ethylene copolymer product having adensity of 0.918, a melt index of 1.0, and a sticking temperature of104° C. The reaction was started by feeding catalyst to a reactorprecharged with a bed of granular resin similar to the product to bemade. The catalyst was a mixture of 5.5 parts titanium tetrachloride,8.5 parts magnesium chloride and 14 parts tetrahydrofuran deposited on100 parts Davison grade 952 silica which had been dehydrated at 800° C.and treated with four parts triethylaluminum prior to deposition and wasactivated with thirty five parts tri-n-hexyl aluminum subsequent todeposition. Prior to starting catalyst feed, the reactor and resin bedwere brought up to the operating temperature of 85° C., were purged ofimpurities by circulating nitrogen through the resin bed. Ethylene,butene and hydrogen concentrations were established at 53, 24, and 11%respectively. Cocatalyst was fed at a rate of 0.3 parts triethylamunimumper part of catalyst.

Reactor start-up was normal. After producing product for 29 hours andequivalent to 61/2 times the weight of the fluidized bed, temperatureexcursions of 1° to 2 C. above bed temperature were observed usingthermocouples located just inside the reactor wall at an elevation of1/2 reactor diameter above the gas distributor plate. Prior experiencehad shown that such temperature excursions are a positive indicationthat sheets of resin are being formed in the fluidized bed.Concurrently, bed voltage (measured using an electrostatic voltmeterconnected to a 1/2 inch diameter spherical electrode located one inchfrom the reactor wall at an elevation of 1/2 reactor diameter above thegas distributor plate) increased from a reading of approximately +1500to +2000 volts to a reading of over +5000 volts and then dropped back to+2000 volts over a 3 minute period. Temperature and voltage excursionscontinued for approximately 12 hours and increased in frequency andmagnitude. During this period, sheets of fused polyethylene resin beganto show up in the resin product. Evidence of sheeting became moresevere, i.e. temperature excursions increased to as high as 20° C. abovebed temperature and stayed high for extended periods of time and voltageexcursions also became more frequent. The reactor was shut down becauseof the extent of sheeting.

EXAMPLE 2

The fluidized bed reactor used in Example 1 was started up and operatedto produce a linear low density ethylene copolymer suitable forextrusion or rotational molding and having a density of 0.934, a meltindex of 5 and a sticking temperature of 118° C. The reaction wasstarted by feeding catalyst similar to the catalyst in Example 1 exceptactivated with 28 parts tri-n-hexylaluminum, to the reactor prechargedwith a bed of granular resin similar to the prroduct to be made. Priorto starting catalyst feed the reactor and resin bed were brought up tothe operating temperature of 85° C. and were purged of impurities withnitrogen. The concentrations of ethylene (52%), butene (14%), hydrogen(21%) were introduced into the reactor. Cocatalyst triethylaluminum wasfed at 0.3 parts per part of catalyst. The reactor was operatedcontinuously for 48 hours and during that period produced resinequivalent to 9 times the amount of resin contained in the bed. Afterthis 48 hour period of smooth operation, sheets of fused resin began tocome out of the reactor with the normal, granular product. At this timevoltages measured 1/2 reactor diameter above the distributor plateaveraged +2000 volts and ranged from 0 to +10,000 volts, while the skinthermocouples at the same elevation indicated excursions of >15° C.above the bed temperature. Two hours after the first sheets were notedin the product from the reactor, it was necessary to stop feedingcatalyst and cocatalyst to the reactor to reduce the resin productionrate because sheets were plugging the resin discharge system. One hourlater, catalyst and cocatalyst feeds were restarted. The production ofsheets continued and after two hours catalyst and cocatalyst feed wereagain stopped and the reaction was terminated by injecting carbonmonoxide. The voltagee at this time was >+12,000 volts and thethermocouple excursions continued until the poison was injected. Intotal, the reactor was operated for 53 hours and produced 101/2 bedvolumes of resin before the reaction was stopped due to sheeting.

EXAMPLE 3

The reactor of Examples 1 and 2 was treated as follows: The treatmentcomprised charging a bed of granular resin and purging and drying thebed with high purity nitrogen to a vapor water concentration of lessthan 10 ppmv. The bed was thereafter fluidized by circulating nitrogen.Chromocene [bis(cyclopentadienyl)chromium] in toluene solution wasinjected into the bed. 4.3×10⁻⁵ pound moles of chromocene were added foreach square foot of steel surface in the system. The bed was heated to92° C. and nitrogen was circulated for 24 hours. After the treatment wascompleted, the bed was cooled to 40° C. and 20 standard cubic feet ofair was injected for each pound of chromocene in the system to oxidizethe chromocene before removing the resin from the reactor.

The treated reactor was then charged with a bed of resin similar to thatdescribed in Example 1. The bed was brought up to 85° C., purged, andthe ethylene, butene, hydrogen, and cocatalyst concentrations wereestablished at the same concentrations of Example 1, prior to injectionof the same catalyst as Example 1. The reactor started up at operatingconditions designed to produce a film grade low density polyethylenecopolymer product having a density of 0.918, a melt index of 1.0, and asintering temperature of 104° C. as in Example 1. The reactor ran for 90hours, producing approximately 3 times as much product as in Example 1,after which it was shut down for routine inspection and maintenance. Notemperature excursions were noted and no resin sheets were formed. Atthe end of the run, the voltage measured near the wall at an elevation1/2 reactor diameter above the gas distributor plate had stabilized atabout -100 volts and no major voltage excursions were observed at anytime during the run.

EXAMPLE 4

The reactor utilized for Example 3 was subsequently charged with a bedof resin similar to that described in Example 2. The bed was heated to90° C., purged and the ethylene (51%), butene (13%) and hydrogen (18%)and cocatalyst (0.3 parts per part of catalyst) were established priorto injection of catalyst. The reaction started smoothly and producedlinear low-density polyethylene resin with a density of 0.934, a meltindex of 5, and a sintering temperature of 118° C.

The reactor operated continuously for 80 hours and produced resinequivalent to twenty times the weight of the resin bed before it wastransitioned to another product grade. The thermocouples located nearthe surface of the reactor wall 1/4 and 1/2 the reactor diameter abovethe distributor showed a few brief (1 minutes) temperature excursions.Voltage measured near the wall at an elevation of 1/2 reactor diameterabove the gas distributor plate averaged +1200 volts and showed voltageoscillation from 0 to as high as +8,000 volts. Some pieces of resintypically 1/4 by 1 by 1 inch with the appearance of sintered fineparticles appeared in the product discharged tank and constituted ≦0.01percent of the resin produced. These did not reduce the production rateof the reaction system nor did they harm the quality of the resinproduced.

As will be seen from the above, the following data corresponds to the Vcformula expressed previously:

    ______________________________________                                        Vc =      -8000 - 50 (sintering temp.) + 90                                             (hydrogen concentration) + 150 (operating                                     temperature)                                                        =         -8000 - .50(118° C.) + 90(18%) + 150(90° C.)          =         + 1220 volts                                                        ______________________________________                                    

EXAMPLES 5-8

Four runs were made utilizing the reactor and procedure of Examples 1and 2 to determine critical voltage. Various ethylene, butene-1copolymers and/or ethylene homopolymers were used for each run as shownin Table I.

The critical voltages, Vc, were the voltage level measured near thereactor wall (one half the reactor diameter above the distributor plate)when the reactor showed symptons of initiation of sheeting (normallysmall skin thermocouple excursions above bed temperature). The stickingtemperatures were estimated from tests in which a reaction wasterminated, the bed allowed to settle for 15 minutes and thenrefluidized.

The results are indicated in Table I below.

                                      TABLE I                                     __________________________________________________________________________              Ethylene                                                                             Butene Catalyst                                                                           Resin                                                 M.sub.2                                                                            Concentra-                                                                           Concentra-                                                                           of   Melt                                                                              Resin                                                                              Sintering                                                                          Operating                                                                           Vc                           Example                                                                            Mole %                                                                             tion Mole %                                                                          tion Mole %                                                                          Example                                                                            Index                                                                             Density                                                                            Temp. °C.                                                                   Temp. °C.                                                                    Volts                        __________________________________________________________________________    5    11   53     24     1    1.0 .918 104  85    +200 to +1000                6    14   51     23     1    2.0 .918 102  85    +200 to +1000                7    30   50     7      2    12. .926 108  85    +2100                        8    21   65     0      2    7.5 .965 125+ 110   +4100                        __________________________________________________________________________    As will be noted from Table I, for Example 5, sheeting begins to occur        over +1000 volts. Moreover, from the above Table I it will be seen that       the critical voltage is dependent on the resin sintering temperature, the     operating temperature and the hydrogen concentration in the recycle gas.  

What is claimed is:
 1. In method for polymerization of one or morealpha-olefins in a gas fluidized bed reactor in the presence of acatalyst prone to cause sheeting, the improvement which comprisesmaintaining the static electric charge in said reactor at the site ofpossible sheet formation below static voltage levels which couldotherwise cause sheet formation.
 2. An improvement in a method for thepolymerization of one or more alpha-olefins in a gas fluidized bedreactor in the presence of a catalyst prone to cause sheeting duringsaid polymerization, the improvement comprising maintaining the staticelectric charge in said reactor at the site of possible sheet formationbelow static voltage levels which would otherwise cause sheet formation,by introducing a chromium containing compound into said reactor in suchmanner as to contact the surfaces of said reactor, said chromium in saidchromium containing compound being present in a valence state of 2 or 3.3. A process for reducing sheeting during production of polyolefins bypolymerization of one or more alpha-olefins in a gas fluidized bedreactor in the presence of a catalyst prone to cause the sheeting whichcomprises contacting the surfaces of the reaction vessel in which saidpolymerization takes place with a chromium containing compound, saidchromium being present in said compound at a valence state of 2 or 3,said chromium containing compound being used in an amount sufficient toreduce the amount of sheeting formed during said polymerization.
 4. Aprocess according to claim 3 in which said chromium containing compoundis dissolved in an inert solvent prior to introduction to said reactor.5. A process according to claim 4 wherein said inert solvent is toluene.6. A process according to claim 4 wherein said inert solvent isintroduced into said reaction vessel in an inert atmosphere.
 7. Aprocess according to claim 3 wherein said chromium containing compoundis bis(cyclopentadienyl) chromium.
 8. A process according to claim 3wherein said polyolefin is a homopolymer of ethylene or a copolymer of≧90% of ethylene, and ≦10% of one or more C₃ to C₈ alpha olefins.
 9. Aprocess according to claim 3 wherein said polyolefin is a homopolymer orpropylene or a copolymer of propylene and one or more of ethylene,butene-1, pentene-1, hexene-1, 4-methylpentene-1, heptene-1 or octene-1.10. A process for reducing sheeting during polymerization of one or morealpha-olefins in a gas fluidized bed reactor to produce a homopolymer ofethylene or a copolymer of ≧90% of ethylene, and ≦10% of one or more C₃to C₈ alpha olefins in the presence of a catalyst prone to causesheeting which comprises introducing a compound containing chromium at avalence state of 2 or 3 dissolved in an inert solvent into said reactorvessel prior to polymerization, said compound being introduced by apressurized inert gas.